This disclosure relates to aircraft and aerial vehicles, in general, and in particular, to a solar powered aerial vehicle that is capable of continuous flight at high latitudes during the winter for extended periods without landing or refueling.
During the past 30-40 years, several efforts have been made to develop solar powered aerial vehicles, primarily unmanned, that are capable of achieving flight durations of from several months to years without needing to land or refuel. These vehicles typically employ solar cells located on their exterior surfaces, primarily their wing surfaces, to capture solar energy during the day. They then use some of that energy to fly during the day, and store excess energy in an energy storage device, e.g., a battery, flywheel, fuel cell, or other storage device, to power the vehicle during the dark of night, when the sun is not available for collection of power.
Aerial vehicles that are capable of operating only near the equator or in the long daylight hours of the summer months have only limited operational value. Accordingly, the critical design conditions for solar powered aerial vehicles become the winter months at high latitudes, where the days are relatively short, the sun is relatively low on the horizon, and the nights are relatively long.
Prior solar powered aerial vehicle designs are significantly limited by the vehicle's limited surface areas and surface norms, i.e., the wings typically comprise the largest vehicle surface available for the collection of solar energy, and are typically disposed horizontally relative to the Earth. This arrangement can be disadvantageous, especially in the winter at high latitudes where the angle of the sun is low. Thus, more vertically oriented solar array surfaces are more desirable in wintertime, low-angle sun conditions for maximum solar energy collection. Moreover, the tactic of simply adding more wing, fuselage or tail surface area beyond the minimum needed for the vehicle to fly so as to collect additional solar energy therewith rapidly encounters a point of diminishing returns. This is because the additional surface area results in additional vehicle weight and drag that require more energy to fly than the additional solar energy collected by the added surface area. Higher efficiency and lighter solar cells and energy storage devices could theoretically solve the problem, but the technologies necessary do not exist currently, and are not projected to exist for the next several decades.
Existing solar powered aerial vehicles include the “HELIOS” experimental unmanned aerial vehicle (UAV) developed by AeroVironment and NASA in the 1990's through the early 2000's. The vehicle was an all-wing design that used existing technology solar cells to collect solar energy. HELIOS flew only during the daytime (with limited operation at night using batteries), and never flew a full day-night cycle. The research vehicle was designed to store energy using a fuel cell energy storage system, but that system was never added to the vehicle. Analyses show that aerial vehicles incorporating the HELIOS technology would not be capable of operation at northern latitudes in the winter without solar cell and power storage efficiencies that are not expected to exist for some decades in the future.
The Boeing Company developed a Solar Powered Formation Flight (SPFF) aerial vehicle concept in the mid-1990's in which multiple aerial vehicles would fly in close, drag-reducing formations, thereby significantly reducing drag, and therefore, the power needed to fly. This concept significantly reduced the technology needs of solar cells and power storage, but added the technology and operational constraints of formation flight that some potential users found undesirable. The SPFF vehicles were actually built and flown, but these did not incorporate solar powered propulsion systems.
QinetiQ has recently developed the “Zephyr,” a small solar powered aerial vehicle with a conventional wing, body and tail design, which is scheduled to demonstrate high altitude, multi-week “24/7” flight, but in summertime conditions and at only moderate northern latitudes. However, this vehicle lacks the capability of solar powered flight in northern latitudes during the winter months.
Other solar powered aerial vehicles have been developed over the past 20 or more years, but none has a documented capability of long-term operation in the critical conditions of northern latitudes and during the winter.
The prior art vehicles described above vary from a highly efficient, all-wing aerial vehicle (HELIOS), to more aggressive, formation-flight technology (SPFF), as well as other, more conventional wing-body-tail, single aerial vehicle designs. However, without significant increases in solar cell and power storage technologies over what is planned for development in the next several decades, these vehicles all lack the capability of year-round flight at northern latitudes during the winter months, mainly due to a shortage of solar energy collection capability.
A long-felt but as yet unsatisfied need therefore exists for aerial vehicles having vertical surfaces or elevation-tracking surfaces that can be oriented almost vertically in winter conditions and at high latitudes and thereby enable the collection of significantly more solar energy at this critical time of year and latitude condition.